Forty years ago, exciting advances in neuroscience, biomaterials, electronics and miniaturization fueled development of the cochlear implant, the world's first and most successful auditory neuroprosthesis. Today, continued scientific advances have set the stage for the next era in neuroprosthetics - that of the direct Brain-Machine Interface (BMI). Preliminary evidence suggests that direct electrical stimulation of the human auditory cortex can induce auditory perceptions. However, it is not yet clear whether such perceptions can form the basis of a working auditory prosthesis for some deaf patients who are unable to benefit from a cochlear implant. In order to evaluate this possibility, we propose technological developments to create a specialized electrode array that would allow for human experimentation in this area. Our study has 2 specific aims summarized as: 1) To use computer-aided design and analysis to create effective models for the study of flexible electrodes and to then manufacture flexible brain electrodes based on the models and test them in viscoelastic materials or in brain tissue, and 2) To evaluate the biological performance of optimized devices in extensive rat studies followed up by a monkey study. In order to be employed in human studies, the new electrode array must be able to access Heschl's gyrus (site of auditory cortex in humans), must have multiple electrode sites and must provide safe and non-damaging electrical activation of the cortical tissue. The specific aims are designed to demonstrate these device specification. We will use computer models to optimize the electrode characteristics and then evaluate optimized electrode arrays via saline soak tests and electrical/mechanical charecterization experiments on the laboratory bench-top. We will then implant refined versions of the electrode arrays into rats and a monkey to test their effectiveness in vivo. To prepare for later human work, we will use cadaver brains to obtain average size and shape of the human Heschl's gyrus and provide a test model for inserting the electrode array. The proposed work represents the application of a fundamental bioengineering approach to a traditional and as yet unsolved biological problem of creating long-lasting cortical interfaces. It represents a new direction in the development of auditory prosthesis systems, and has implications for basic neuroscience while providing hope for those deaf patients unable to benefit from a cochlear implant but interested in a prosthesis.